The present invention relates to a method for the flattening and cleaning treatment of the surface of a single crystal of semiconductor silicon, which is widely used as a substrate of various kinds of electronic devices. In particular, the invention relates to a method for the treatment of removal of the surface contamination and the atomic scale corrugation at the same time, which is formed by the removal of the oxidized surface film on single crystal silicon surface oriented toward (001) crystallographic direction.
In recent semiconductor industry, almost all the electronic devices currently under use in various kinds of electronic instruments are formed on the substrate of a single crystal of semiconductor silicon. Among these electronic devices, central processing units or integrated circuits such as so-called microchips and memory devices are produced by forming an extremely fine network of electronic circuit on the (001) surface of a silicon single crystal wafer. Along with the trend in recent years toward finer and finer patterning of the electronic circuits, the requirements for the flatness and cleanness of the silicon wafer surfaces are increasing year by year.
For example, in the case of a 4-megabyte DRAM, which is a widely used memory device in present days, the smallest line-width of the circuit pattern to be formed on the (001) surface of a silicon single crystal wafer is about 0.5 .mu.m. However the required fineness to form more integrated circuit is expected to decrease down to about 0.25 .mu.m early in the coming new century. In order to accomplish such extreme fineness of the circuit network, the flatness of the (001) surface of a silicon wafer is further improved and any trace of contamination is avoided.
As a general method for cleaning of the surface of a silicon wafer, first the surface of the silicon wafer is oxidized to form an oxidized surface film, which is then removed by dissolving away with a hydrofluoric acid solution, and at the same time the surface free bonds of the outermost silicon atoms to be exposed by the oxide removal are terminated by the hydrogen atoms. The surface of a silicon wafer as formed is inherently covered by an oxidized surface film and, when the oxidized surface film is removed, the bare silicon surface is highly active so that the bare surface is immediately oxidized by the oxygen in the atmospheric air and contaminant materials from atmospheric air adsorb to the surface resulting in an oxidized and contaminated surface.
This problem can be solved at least partly by undertaking the method as mentioned above. Dissolving the oxidized surface film by the hydrofluoric acid and at the same time terminating the surface silicon bonds by the hydrogen atoms, the silicon surface becomes resistive against oxidation and the adsorption of the contaminants even in atmospheric condition because of the extremely strong bonding between the silicon and the hydrogen atoms.
However there are two problems about the conventional treatment method by using a hydrofluoric acid solution. First, the (001) surface of the silicon is subject to etching by the hydroxyl ions contained in the hydrofluoric acid solution. Second, the (001) surface is oxidized by the oxygen dissolved and contained in the hydrofluoric acid solution during the oxide removal process and the oxidized parts by the dissolved oxygen are to be removed by the hydrofluoric acid again. Hence the flatness of the resulting (001) surface decreases because of the surface etching with the process described above. Furthermore, in the cleaning and hydrogen termination treatment for the (001) surface by using hydrofluoric acid solution, it is unable to avoid that a surface strain is produced due to the repulsive force between the hydrogen atoms in the dihydride molecules formed on the surface or the intermolecular interaction between the precursor molecules formed on the surface during the removal process of the oxidized surface film. These strain contained in the silicon surface enlarge the surface energy but, in addition, the surface energy is effectively reduced by the relaxation due to the roughness of the (001) silicon surface. When the silicon surface is flattened by removing the roughness, strain is unevenly accumulated over the surface and the areas with accumulated strain enlarge the surface energy than the areas free from strain. Then, the area containing strain is easily attacked by the hydrofluoric acid, hydroxyl ions and molecular oxygen dissolved in the solution, and resulting surface flatness decreases.
Thus, the treatment method described above is only applicable to the flattening and cleaning of the only limited area of the (001) silicon surface, and there have existed no effective ways of the surface flattening and cleaning treatment over the whole (001) silicon surface.